1. Field of the Invention
This invention relates to foils and a method of reducing drag on a foil.
2. Brief Description of the Related Art
A foil to which this invention relates may be used on a lifting body on aircraft, missiles, ships, boats, vehicles and on any other equipment that uses any form of foil, including propellers, turbine and compressor blades and fans.
One of the main aims of foil research and development is to reduce drag, that is the resistant force acting in a direction opposite to the direction of the foil motion. Drag on a foil may be reduced by controlling the flow over the foil. This can be achieved using various forms of boundary layer control, for example surface finish, surface shape, various forms of vortex generator and boundary layer suction or blowing.
Further reductions may be achieved by reducing the effects of induced drag, which is associated with the development of lift, and results in the formation of a single vortex in the trailing flow from the foil. In conventional practice induced drag can be kept to a low value by providing a large ratio of foil length to foil width.
DE-A-3 127 257 discloses a method of reducing the induced drag of a lift generating aerofoil by disposing additional members at discrete positions on the wing in the region of free turbulent flow occurrence and using these members to generate artificial turbulent flows which have a direction of rotation opposed to the direction of rotation of the free turbulent flow. The additional members have a swirl generating adjustable air guidance system therein operable to vary the circulation or twist of air emerging therefrom. The intensity of the artificial turbulent flow is controlled so that it is at least approximately equal to that of the free turbulent flow in the same wing spread position. Hence, the artificial turbulent flow compensates for the free turbulent flow so that the induced drag on the foil is reduced to a minimum.
U.S. Pat. No. 3,369,775 discloses an aircraft wing system having means to overcome induced drag by providing a spanwise distribution of compensating trailing vortices equal in magnitude but opposite in sign to that produced by the wing lifting system. Several such means are disclosed, for example a spanwise distribution of vanes, ridges, grooves or fences may be provided on the surface of the wing itself. A jet flap system is also described. The function of the compensating vortices is to reduce or avoid the shedding of wing trailing vorticity, and thereby reduce the induced drag on the wing.
U.S. Pat. No. 2,743,888 discloses the provision of winglets at the tip of a wing for an aircraft, the winglets being designed to reduce the losses due to the tip vortex that is formed as a result of the airflow around the tip of the wing, by splitting the single tip vortex into a series of two or more vortices. It teaches the provision of retractable winglets in the leading edge of a delta wing at locations in the leading edge that are spaced apart in the spanwise direction along the leading edge. These are to be extended during take-off and landing and are to be retracted at other times such as cruise and in high speed flight. It is understood that these retractable winglets in the leading edge are proposed for splitting the detached vortex flow that would leave the wing from the leading edge during take-off and landing, the delta wing being designed to use vortex lift of which such detached vortex flow is a characteristic. Use of winglets at the tip of a wing, as proposed in U.S. Pat. No. 2,743,888, has been tested extensively and found not to reduce induced drag. Current theory suggests that this failure is a result of the fact that it is not possible to effectively prevent a single tip vortex from forming, in other words that the winglets do not achieve the purpose of splitting the tip vortex for which they were provided.